1. Technical Field of the Invention
The present invention relates to an optical waveguide circuit, optical branched waveguide circuit, and optical modulator. More particularly, the invention relates to an optical waveguide circuit, optical branched waveguide circuit, and optical modulator which are used for optical wiring parts of optical communications, vehicles, etc. or optical signal processing, optical control, and optical measurement of industrial equipment, have a small size, and can be easily fabricated.
2. Description of the Related Art
Due to advances in optical communications technology and optical signal processing technology, increasing use is being made of optical wiring in vehicles via optical fibers, optical measurement for controlling various types of industrial equipment using optical signals, optical signal processing, etc. Optical waveguide circuits, optical branched circuits, optical branched waveguide circuits, and optical modulators are being used in these fields.
First, a description will be given of an optical branched waveguide used in a Mach-Zehnder type optical modulator.
FIG. 1 is a view of the configuration of a Mach-Zehnder type optical modulator of the related art. A Mach-Zehnder type optical modulator 90 illustrated in FIG. 1 is constituted by an input waveguide 92 formed in a substrate 91, Y-branch type waveguides 93.sub.1 and 93.sub.2, linear waveguides 94.sub.1 and 94.sub.2, Y-branch type waveguides 95.sub.1 and 95.sub.2, an output waveguide 96, and electrodes 97.sub.1 to 97.sub.4 provided on the two sides of the linear waveguides 94.sub.1 and 94.sub.2.
Light is emitted to the input waveguide 92 and is branched at the Y-branch type waveguides 93.sub.1 and 93.sub.2. The branched lights are propagated through the linear waveguides 94.sub.1 and 94.sub.2. In the process, the lights are modulated in response to the voltages applied to the Y-branch type waveguides 93.sub.1 and 93.sub.2. The modulated lights pass through the Y-branch type waveguides 95.sub.1 and 95.sub.2 and are combined at the output waveguide 96. The result is output from an end portion of the output waveguide 96. The lights propagated through the linear waveguides 94.sub.1 and 94.sub.2 are changed in phase by an electro-optic effect by applying an electric field based on the modulation signal to the linear waveguides 94.sub.1 and 94.sub.2 via the electrodes 97.sub.1 to 97.sub.4. The intensity of the output interference light changes in response to this change in phase for modulation of the light.
The input waveguide 92 and the Y-branch type waveguides 93.sub.1 and 93.sub.2 in the Mach-Zehnder type optical modulator 90 constitute the optical branched waveguide illustrated in FIGS. 2 and 3.
The Mach-Zehnder type optical modulator illustrated in FIG. 1 encounters the disadvantages due to the restrictions of the dimensions of the optical branched waveguide illustrated in FIGS. 2 and 3 etc.
The optical branched waveguide illustrated in FIGS. 2 and 3 suffers from the disadvantage of large dimensions and difficulty of reduction of size. As a result, the Mach-Zehnder type optical modulator illustrated in FIG. 1 also suffers from the disadvantage of enlargement of size.
The disadvantage of the inability of the optical branched waveguide to be reduced in size will be considered next.
In the optical modulator, the propagated light must be of a single mode, therefore the input waveguide 92 and the Y-branch type waveguides 93.sub.1 and 93.sub.2 constituting the modulator are restricted in design by the length of the branched light path. For example, in the case of an optical branched waveguide using lithium niobate, a branch angle .theta. of the Y-branch type waveguides 93.sub.1 and 93.sub.2 for branching while maintaining the single mode is very small, i.e., about 1.degree.. As a result, the input waveguide 92 and the Y-branch type waveguides 93.sub.1 and 93.sub.2 must be almost linearly connected.
Further, the difference of the refractive indexes of a core and a cladding of the waveguide is made small to obtain a single mode waveguide, therefore, in order to draw waveguides having a slope with a branch angle .theta. of 1.degree. parallel to the input waveguide 92 perpendicular to the end face of the rectangular substrate, to prevent the leakage of light to the outside of the waveguides, the radius of curvature R of the waveguides must be set to 50 mm or more and the curved portion a must be made 5 to 10 mm.
Further, at the connection portion of the Y-branch type waveguides 93.sub.1 and 93.sub.2, the waveguide width becomes double when viewed from the input waveguide 92 side. If the waveguides are simply connected, the transmission loss is increased due to the reflection of light or conversion of the transmission mode. For this reason, an excess length b of several millimeters becomes necessary for tapering the connection portion and smoothly converting the transmission mode.
Further, in order to send the optical signal to the input waveguide 92, a linear waveguide c of a predetermined length or more, for example, 3 mm or more, becomes necessary to reduce the disturbance in the transmission mode at the connection point.
When all these are added up, the entire length of the Y-branch portion becomes about 15 mm or more.
In the Mach-Zehnder type optical modulator 90, it is necessary to provide two sets of such Y-branch type waveguides facing to each other and further provide the linear waveguides 94.sub.1 and 94.sub.1 for application of the electric field between them. As a result, the entire modulator has a length of 40 to 50 mm or more. This dimension is very large considering the fact that most semiconductor devices undergoing the same type of process are less than 1 mm square in size.
With a modulator having such a length, when considering the machining after this and deformation of the parts, the width of the substrate must be made at least 6 mm--resulting in a part with a very large dimension.
In this way, in the Mach-Zehnder type optical modulator, the reduction of size is difficult. Since the size is large, the number of devices which can be obtained from one substrate is small compared with usual semiconductor devices. For example, 10 or less Mach-Zehnder type optical modulators 90 shown here can be obtained from a 3-inch square substrate generally used in the fabrication of optical parts. This is very small when compared with many semiconductor devices made using a similar process. As a result, the Mach-Zehnder type optical modulator of the related art suffers from the disadvantages that the productivity is low and the cost can not be reduced.
Further, the Mach-Zehnder type optical modulator also suffers from the disadvantages that its structure is complex and therefore production is difficult. The shape of the connection portion of the Y-branch portion of the Mach-Zehnder type optical modulator 90 has an influence upon the branch characteristics and loss, therefore the portion must be very precisely produced. Particularly, a machining precision of about 0.1 .mu.m becomes necessary for the center conically shaped portion of the branch. As a result, very high precision machining becomes necessary, and there arises the disadvantage of a further increase of costs.
While the optical branched waveguide was explained above in relation to the Mach-Zehnder type optical modulator 90, the same disadvantages are encountered even when viewing an optical branched waveguide alone.